Use EVFILT_SIGNAL for kqueue latches.

macdice · macdice · commit 6148656a0be1 · 2021-03-01T14:20:04.000+13:00
Cut down on system calls and other overheads by waiting for SIGURG explicitly with kqueue instead of using a signal handler and self-pipe. Affects *BSD and macOS systems. This leaves only the poll implementation with a signal handler and the traditional self-pipe trick. Discussion: https://postgr.es/m/CA+hUKGJjxPDpzBE0a3hyUywBvaZuC89yx3jK9RFZgfv_KHU7gg@mail.gmail.com
diff --git a/src/backend/storage/ipc/latch.c b/src/backend/storage/ipc/latch.c
@@ -18,6 +18,8 @@
  * don't need to register a signal handler or create our own self-pipe.  We
  * assume that any system that has Linux epoll() also has Linux signalfd().
  *
+ * The kqueue() implementation waits for SIGURG with EVFILT_SIGNAL.
+ *
  * The Windows implementation uses Windows events that are inherited by all
  * postmaster child processes. There's no need for the self-pipe trick there.
  *
@@ -150,7 +152,7 @@ static volatile sig_atomic_t waiting = false;
 static int	signal_fd = -1;
 #endif
 
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 /* Read and write ends of the self-pipe */
 static int	selfpipe_readfd = -1;
 static int	selfpipe_writefd = -1;
@@ -189,7 +191,7 @@ static inline int WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 void
 InitializeLatchSupport(void)
 {
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 	int			pipefd[2];
 
 	if (IsUnderPostmaster)
@@ -277,6 +279,11 @@ InitializeLatchSupport(void)
 		elog(FATAL, "signalfd() failed");
 	ReserveExternalFD();
 #endif
+
+#ifdef WAIT_USE_KQUEUE
+	/* Ignore SIGURG, because we'll receive it via kqueue. */
+	pqsignal(SIGURG, SIG_IGN);
+#endif
 }
 
 void
@@ -300,7 +307,7 @@ InitializeLatchWaitSet(void)
 void
 ShutdownLatchSupport(void)
 {
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 	pqsignal(SIGURG, SIG_IGN);
 #endif
 
@@ -310,7 +317,7 @@ ShutdownLatchSupport(void)
 		LatchWaitSet = NULL;
 	}
 
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 	close(selfpipe_readfd);
 	close(selfpipe_writefd);
 	selfpipe_readfd = -1;
@@ -335,7 +342,7 @@ InitLatch(Latch *latch)
 	latch->owner_pid = MyProcPid;
 	latch->is_shared = false;
 
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 	/* Assert InitializeLatchSupport has been called in this process */
 	Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
 #elif defined(WAIT_USE_WIN32)
@@ -399,7 +406,7 @@ OwnLatch(Latch *latch)
 	/* Sanity checks */
 	Assert(latch->is_shared);
 
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 	/* Assert InitializeLatchSupport has been called in this process */
 	Assert(selfpipe_readfd >= 0 && selfpipe_owner_pid == MyProcPid);
 #endif
@@ -611,7 +618,7 @@ SetLatch(Latch *latch)
 		return;
 	else if (owner_pid == MyProcPid)
 	{
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 		if (waiting)
 			sendSelfPipeByte();
 #else
@@ -898,13 +905,15 @@ AddWaitEventToSet(WaitEventSet *set, uint32 events, pgsocket fd, Latch *latch,
 	{
 		set->latch = latch;
 		set->latch_pos = event->pos;
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 		event->fd = selfpipe_readfd;
 #elif defined(WAIT_USE_EPOLL)
 		event->fd = signal_fd;
 #else
 		event->fd = PGINVALID_SOCKET;
+#ifdef WAIT_USE_EPOLL
 		return event->pos;
+#endif
 #endif
 	}
 	else if (events == WL_POSTMASTER_DEATH)
@@ -1125,6 +1134,18 @@ WaitEventAdjustKqueueAddPostmaster(struct kevent *k_ev, WaitEvent *event)
 	AccessWaitEvent(k_ev) = event;
 }
 
+static inline void
+WaitEventAdjustKqueueAddLatch(struct kevent *k_ev, WaitEvent *event)
+{
+	/* For now latch can only be added, not removed. */
+	k_ev->ident = SIGURG;
+	k_ev->filter = EVFILT_SIGNAL;
+	k_ev->flags = EV_ADD;
+	k_ev->fflags = 0;
+	k_ev->data = 0;
+	AccessWaitEvent(k_ev) = event;
+}
+
 /*
  * old_events is the previous event mask, used to compute what has changed.
  */
@@ -1156,18 +1177,21 @@ WaitEventAdjustKqueue(WaitEventSet *set, WaitEvent *event, int old_events)
 		 */
 		WaitEventAdjustKqueueAddPostmaster(&k_ev[count++], event);
 	}
+	else if (event->events == WL_LATCH_SET)
+	{
+		/* We detect latch wakeup using a signal event. */
+		WaitEventAdjustKqueueAddLatch(&k_ev[count++], event);
+	}
 	else
 	{
 		/*
 		 * We need to compute the adds and deletes required to get from the
 		 * old event mask to the new event mask, since kevent treats readable
 		 * and writable as separate events.
 		 */
-		if (old_events == WL_LATCH_SET ||
-			(old_events & WL_SOCKET_READABLE))
+		if (old_events & WL_SOCKET_READABLE)
 			old_filt_read = true;
-		if (event->events == WL_LATCH_SET ||
-			(event->events & WL_SOCKET_READABLE))
+		if (event->events & WL_SOCKET_READABLE)
 			new_filt_read = true;
 		if (old_events & WL_SOCKET_WRITEABLE)
 			old_filt_write = true;
@@ -1620,11 +1644,8 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 		occurred_events->events = 0;
 
 		if (cur_event->events == WL_LATCH_SET &&
-			cur_kqueue_event->filter == EVFILT_READ)
+			cur_kqueue_event->filter == EVFILT_SIGNAL)
 		{
-			/* There's data in the self-pipe, clear it. */
-			drain();
-
 			if (set->latch && set->latch->is_set)
 			{
 				occurred_events->fd = PGINVALID_SOCKET;
@@ -1999,7 +2020,7 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 }
 #endif
 
-#if defined(WAIT_USE_POLL) || defined(WAIT_USE_KQUEUE)
+#if defined(WAIT_USE_POLL)
 
 /*
  * SetLatch uses SIGURG to wake up the process waiting on the latch.
